Silicon solar cells are manufactured using amorphous and polycrystalline solar-grade substrates. Epitaxial silicon substrates have fewer crystal defects than amorphous and polycrystalline substrates and therefore can be used to manufacture higher efficiency solar cells; however, epitaxial silicon substrates are expensive to produce using conventional epitaxial deposition equipment. Conventional epitaxial deposition equipment used to manufacture prime-grade silicon substrates for integrated devices have high operating costs and are not suitable for solar-grade substrate manufacturing where the cost per substrate is paramount.
Commercially viable kerf-less crystalline silicon substrate manufacturing for solar cell production requires low equipment capital expense, high energy efficiency, and high process gas utilization. Conventional lamp-heated crossflow epitaxial deposition equipment designs do not comply with capital expense, energy efficiency, and process gas utilization requirements for kerf-less crystalline silicon solar substrate manufacturing. For example, the inventors have observed that single substrate systems are: throughput-limited; less than twenty-percent energy efficient; optimized for low gas efficiency to produce thin (less than one micron thick) films; and unsuitable for depositing thick films which produce unwanted parasitic deposits and require frequent maintenance.
Several attempts have been made to develop and commercialize epitaxial deposition equipment for solar substrate manufacturing. The major deficiencies with these multi-substrate batch and continuous in-line equipment architectures are low energy efficiency and low process gas utilization. These architectural deficiencies are due to the sub-optimal method for substrate heating (i.e. radiative versus conductive or convective heating) and the larger than necessary size of the processing volume.
Conventional epitaxial deposition systems mechanically support and conductively heat one or more substrates using silicon carbide coated graphite or solid silicon carbide susceptors. The susceptor thermal mass is optimized to interact with the substrate and the heat source in order to produce a tunable thermal gradient across the substrate surface. The inventors believe that heating the susceptor using infrared radiation lamps is an energy inefficient approach for heating the susceptor and substrate. Furthermore, the inventors believe that resistively heating the substrates by applying AC or DC electricity to the susceptor to induce Joule heating is a meritless capital intensive approach that would not satisfy low cost of ownership requirements. Direct heating requires an inordinate number of complex electromechanical components, costly power supplies, and one susceptor per substrate; furthermore, direct heating requires one face of the substrate to be in contact with the susceptor which precludes double-sided processing.
Therefore, the inventors have provided embodiments of a substrate processing tool that may provide some or all of high energy efficiency, high precursor utilization, low cost of equipment, and high throughput and process quality.